1. Field of the Invention
The present invention relates to a method and apparatus for correcting the loop gain of a servo loop in a fine adjustment servo system such as a focus servo system and a tracking servo system.
2. Background of the Art
A servo system such as a tracking servo system is used for making an information detecting point accurately follow a recording track of a disk. A focus servo system which always accurately focuses a laser beam as the information detecting point upon the recording face of the disk is a prerequisite for e.g., an optical playback for use in replaying the information recording disk (simply called the disk) including video and digital audio disks. Those servo systems are classified as fine adjustment servo systems as compared with coarse adjustment servo systems such as a slider servo system used for coarsely adjusting a moving object over comparatively large distances. These fine adjustment servo systems limit the deflection of the object being controlled (such as an objective lens or pickup including the objective lens) relative to a target within a finely adjustable range.
In the fine adjustment servo system, however, the loop gain of each servo system may fluctuate because of variations in the power of a light source such as a laser diode disk reflectance and a focus error signal detecting system. The fluctuation of the loop gain may be detrimental to the loop control against focus and tracking errors or may cause oscillation. Accordingly, stable servo operation is unavailable if the loop gain of each servo system fluctuates.